1. Field of the Invention
The present invention relates to a process for removing carbon dioxide (CO.sub.2) from gases such as combustion gases. More particularly, it relates to a process for efficiently removing CO.sub.2 from combustion gases or the like at the atmospheric pressure using an aqueous solution containing a specific amine compound.
2. Description of the Related Art
Heretofore varied approaches have been studied and proposed for the recovery and removal of acid gases, especially CO.sub.2, from various gases (to be treated), such as industrial gases and combustion gases generated in chemical plants handling natural and synthetic gases. With combustion gases, for example, intensive studies are being made for the removal and recovery of CO.sub.2 from the gases through contact with an aqueous alkanolamine solution or for the storage of the recovered CO.sub.2 instead of releasing it to the atmosphere.
In recent years the greenhouse effect of CO.sub.2 has been found partly responsible for the global warming, and counteracting that effect is urgently needed across the world to protect the environment on the earth. The source of CO.sub.2 is omnipresent in the whole areas of human activities that involve the combustion of fossil fuels, and the tendency is toward more and more stringent emission control. In view of these, energetic studies are under way on the removal and recovery of CO.sub.2 from boiler combustion gases, especially from the gases produced by power-generating installations such as steam power plants that burn enormous quantities of fossil fuels, through contact with an aqueous alkanolamine solution or the like or on the storage of the recovered CO.sub.2 without discharge to the atmosphere.
Examples of the alkanolamine are monoethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine (MDEA), diisopropanolamine, and diglycolamine. Usually, monoethanolamine (MEA) is used by preference. Aside from these, the use of secondary and tertiary hindered amines in the form of aqueous solutions is under investigation.
Aqueous solutions of these alkanolamines, typified by MEA, have been used as liquid absorbents for the absorption and removal of CO.sub.2 from combustion gases. However, they have not always proved satisfactory in view of the CO.sub.2 absorption quantity per given quantity of each aqueous amine solution at a given concentration, the CO.sub.2 absorption quantity per unit amine molar amount of the aqueous amine solution at a given concentration, the rate of CO.sub.2 absorption at a given concentration, and the thermal energy required for the regeneration of the spent aqueous alkanolamine solution after the absorption.
As for the separation of acid gases from various mixed gases through the medium of amine compounds, many techniques are known in the art.
Japanese Patent Provisional Publication No. 100180/1978 discloses a process for the removal of acid gas which comprises contacting a normally gaseous mixture with an amine-solvent liquid absorbent consisting of (1) an amine mixture composed of at least 50 mol % of a sterically hindered amine which contains at least one secondary amino group bound to either a secondary carbon or tertiary carbon or a primary amino group bound to a tertiary carbon, forming a part of the ring, and at least about 10 mol % of a tertiary amino-alcohol and (2) a solvent for the amine mixture which serves as a physical absorbent for the acid gas. Examples cited of the sterically hindered amine are 2-piperidineethanol[2-(2-hydroxyethyl)-piperidine] and 3-amino-3-methyl-1-butanol and examples of the solvent are sulfoxide compounds which may contain up to 25 wt % water. As regards the gas to be treated, the reference publication, p.11, upper left col., mentions "a normally gaseous mixture containing carbon dioxide and hydrogen sulfide at high concentrations, e.g., 35% CO.sub.2 and 10-12% H.sub.2 S" and uses CO.sub.2 itself in its working example.
Japanese Patent Provisional Publication No. 71819/1986 describes a composition for acid gas scrubbing which contains a hindered amine and a nonaqueous solvent such as sulfolane. The publication explains the advantage of the hindered amine in the absorption of CO.sub.2, with the aid of a reaction formula.
Chemical Engineering Science, Vol.41, No.4, pp.997-1003, reveals the carbonic acid gas absorption behavior of an aqueous solution of 2-amino-2-methyl-1-propanol (AMP), a hindered amine. As the gases to be absorbed, CO.sub.2 at the atmospheric pressure and a mixture of CO.sub.2 and nitrogen were used.
Chemical Engineering Science, Vol. 41, No. 2, pp. 405-408, reports the rates of CO.sub.2 and H.sub.2 S absorption by the aqueous solutions of hindered amines such as AMP and of straight-chain amines such as MEA at around ordinary temperatures.
U.S. Pat. No. 3,622,267 teaches a technique of purifying a synthetic gas such as partially oxidized gas obtained from crude oil, that contains CO.sub.2 at a high partial pressure, e.g., 30% CO.sub.2 at 40 atm., using an aqueous mixture containing methyldiethanolamine and monoethylmonoethanolamine.
German Patent Provisional Publication 1,542,415 unveils in its published specification a technique of adding a monoalkylalkanolamine or the like to a physical or chemical absorbent so as to enhance the rates of absorption of CO.sub.2, H.sub.2 S, and COS. Similarly, German Patent Provisional Publication 1,904,428 makes public a technique of adding monomethylethanolamine to methyldiethanolamine to increase the latter's absorption rate.
U.S. Pat. No. 336,233 discloses a technique using an aqueous solution of 0.81-1.3 mole piperazine per liter as a scrubbing solution for the purification of natural, synthetic, or gasified coal gas. Alternatively, piperazine in the form of an aqueous solution with a solvent such as methyldiethanolamine, triethanolamine, diethanolamine, or monomethylethanolamine is used as well for the scrubbing purpose.
Japanese Patent Provisional Publication No. 63171/1977 introduces a CO.sub.2 absorbent prepared by adding piperazine or a piperazine derivative such as hydroxyethylpiperazine as a promoter to a tertiary alkanolamine, monoalkylalkanolamine or the like. In reality, the combination of piperazine and monomethylaminoethanolamine as a monoalkylalkanolamine is not tested. The disclosure is directed to the treatment of synthetic gas; it is utterly silent on the removal of CO.sub.2 from combustion gases at atmospheric pressures.
In view of the state of the art described above, there is a demand for a process whereby CO.sub.2 can be efficiently removed from various gases. For, especially when a gas is to be treated with an aqueous solution containing a given concentration of a CO.sub.2 absorbent (amine compound), great significance is attached today to the choice of an absorbent which excels in both the CO.sub.2 absorption quantity per unit mole of the absorbent and the CO.sub.2 absorption quantity and absorption rate per unit volume of the aqueous solution. Also, the advent of an absorbent which consumes less thermal energy than heretofore in separating absorbed CO.sub.2 and regenerating the liquid absorbent, is being awaited.
In some cases where a single amine compound can hardly meet all such desired conditions, it is likely that the discovery of a compound which can satisfy some of the needs can lead to an approach to greater satisfaction of the requirements, for example, through mixing with some other amine compound. Thus, only if the CO.sub.2 absorption quantity per unit mole of the absorbent is high, there will be a possibility of achieving better result through an improvement of the absorption rate in a separate way.